1. Field of the Invention
The present invention relates to an elastic wave device, which is used in a circuit of a communication apparatus or an electronic apparatus to propagate an elastic wave.
2. Description of Related Art
FIG. 1 shows an example of characteristics of a conventional elastic wave device in which lithium niobate (LiNbO3, called LN) is conventionally used. This example is indicated in Published Unexamined Japanese Patent Application H9-167936 (1997) (first literature). In FIG. 1, a Y-axis indicates a propagation loss of a surface acoustic wave (called SAW), and a propagation loss per one wavelength (xcex) of the surface acoustic wave denoting a propagated distance is indicated by a unit of decibel (dB). An X-axis indicates a normalized electrode thickness (h/xcex) normalized by using the wavelength xcex. Here, a symbol h indicates an electrode thickness.
The characteristics shown in FIG. 1 are obtained by setting a propagation direction of the surface acoustic wave to a direction along a crystal X-axis of the lithium niobate and by setting a substrate surface to a plane perpendicular to a xcex8-rotated Y-axis which is obtained by rotating a crystal Y-axis of the lithium niobate by an angle xcex8 around the crystal x-axis. In particular, the rotation angle xcex8 of the crystal Y-axis ranges from 62 to 74 degrees.
FIG. 2 is a cross sectional view of the conventional elastic wave device. In FIG. 2, 1 indicates a lithium niobate (LN) substrate. 2 indicates an electrode which is made of aluminum (Al) and is arranged on the LN substrate 1. As shown in FIG. 2, a plane perpendicular to the xcex8-rotated Y-axis is set to a surface of the LN substrate 1, and the characteristics shown in FIG. 1 are determined in the case where the whole surface of the LN substrate 1 is covered with an electrode material 2 having a thickness h. The electrode 2 is usually made of aluminum (Al). In cases where the plane perpendicular to the xcex8-rotated Y-axis is set to a surface of the LN substrate 1 and the crystal X-axis of the LN substrate 1 is set to the propagation direction of the surface acoustic wave, the LN substrate 1 is expressed by xcex8-rotated Y-cut X-propagation lithium niobate and is abbreviated to xcex8YX-LN or xcex8YX- LiNbO3.
As is apparent in the characteristics shown in FIG. 1, in cases where a cut angle xcex8 (or a rotation angle xcex8) is, for example, equal to 62 degrees, the propagation loss is minimized in the neighborhood of the normalized electrode thickness (h/xcex) set to 0.03. Also, in cases where a cut angle xcex8 is equal to 74 degrees, the propagation loss is minimized in the neighborhood of the normalized electrode thickness (h/xcex) set to 0.1. Therefore, in cases where a surface acoustic wave (SAW) device is manufactured on condition that the normalized electrode thickness (h/xcex) is higher than 0.05, it is realized that a cut angle xcex8 is higher than 66 degrees to minimize the propagation loss. As is described above, the propagation loss can be minimized by selecting an appropriate combination of the normalized electrode thickness (h/xcex) and the cut angle xcex8, and an insertion loss of the SAW device can be reduced.
Here, several types waves other-than the surface acoustic wave are also called the elastic waves. In cases where the propagation direction is set to the X-axis of the LN substrate 1 and the cut angle xcex8 is set to a value ranging from 62 to 74 degrees, a surface skimming bulk wave (SSBW) denoting a type of bulk wave and a leaky surface acoustic wave (LSAW) are propagated though the surface of the LN substrate 1. These SSBW and LSAW are disclosed in a literature: xe2x80x9cPaper of Institute of Electronics and Communication Engineers of Japanxe2x80x9d, 84/1, Vol.J67-C, No.1, pp.158-165 (second literature). However, in this application, the SAW, SSBW and LSAW are generally called the surface acoustic wave SAW except where it is required to distinguish the SAW, SSBW and LSAW from each other.
FIG. 3 is a diagram showing the configuration of a surface acoustic wave (SAW) filter denoting a type of elastic wave device. In FIG. 3, 1 indicates a lithium niobate (LN) substrate functioning as a piezo-electric element. 3 indicates an electrode finger. 4 indicates a bonding pad. 5 indicates an input-side inter-digital transducer (IDT), which is composed of the input-side electrode fingers 3 arranged in a comb-like shape, for performing an energy transformation from electricity to surface acoustic wave. 6 indicates an output-side inter-digital transducer (IDT), which is composed of the output-side electrode fingers 3 arranged in a comb-like shape, for performing an energy transformation from surface acoustic wave to electricity. 7 indicates an input terminal. 8 indicates an output terminal. A length of portions of the electrode fingers 3 crossing each other is called an aperture width W, and a maximum value of the aperture width W is called a maximum aperture width WO.
FIG. 4 is a cross sectional view of the SAW filter shown in FIG. 3. In FIG. 4, a symbol xe2x80x9cwxe2x80x9d indicates an electrode finger width of each electrode finger 3. A symbol xe2x80x9cpxe2x80x9d indicates an arrangement interval of each pair of electrode fingers 3. A symbol xe2x80x9chxe2x80x9d indicates an electrode thickness of each electrode finger 3.
Next, an operation is described.
When an electric signal is supplied to the input terminal 7, an electric field is generated in a crossing area of each pair of electrode fingers 3 of the input-side IDT 5. In this case, because the LN substrate 1 functions as a piezo-electric element, distortion is caused in the LN substrate 1 by the electric field. In cases where the electric signal has a frequency f, the distortion caused in the LN substrate 1 is changed with time to oscillate the LN substrate 1 at the frequency f. Therefore, a surface acoustic wave (SAW) is generated in the LN substrate 1 and is propagated through the LN substrate 1 in a direction perpendicular to a longitudinal direction of the electrode fingers 3. Thereafter, the propagated surface acoustic wave is transformed into an electric signal in the output-side IDT 6. Here, a process, in which an electric signal is transformed into a surface acoustic wave, and a process, in which a surface acoustic wave is transformed into an electric signal, have a reversible relationship with each other.
As shown in FIG. 1, in cases where the cut angle xcex8 is near to 64 degrees and the propagation direction is set to a direction along the X-axis, as is disclosed in the second literature, a displacement component of the surface acoustic wave is parallel to the electrode fingers 3, and the surface acoustic wave has a directional component parallel to the surface of the LN substrate 1. The displacement component of the surface acoustic wave depends on a material of the LN substrate 1, a cut surface of the LN substrate 1, a cut angle xcex8 of the cut surface and a propagation direction of the surface acoustic wave. The surface acoustic wave oscillated in the input-side. IDT 5 is propagated toward the output-side IDT 6. In cases where the LN substrate 1 causes a propagation loss to the surface acoustic wave, an electric power of the surface acoustic wave arriving at the output-side IDT 6 becomes lower than that of the surface acoustic wave obtained just after the oscillation of the LN substrate 1 in the input-side IDT 5. A degree of the propagation loss caused to the surface acoustic wave is almost equal to a value which is obtained by multiplying a standardized distance by a propagation loss per one wavelength shown in FIG. 1. The standardized distance is obtained by standardizing a distance between the center of the input-side IDT 5 and the center of the output-side IDT 6 with respect to the wavelength of the surface acoustic wave.
Therefore, in cases where the distance between the input-side IDT 5 and the output-side IDT 6 is fixed to a constant value, as the propagation loss per wavelength in the LN substrate 1 is increased, an insertion loss of the SAW filter is increased. As is described in a literature: xe2x80x9cSurface Acoustic Wave Technologyxe2x80x9d edited by Institute of Electronics and Communication Engineers of Japan and published by Korona publishing company, November of 1983, pp.56-81 (third literature), the wavelength xcex of the surface acoustic wave is equivalent to double of the arrangement interval p of the electrode fingers 3. Therefore, even though the input-side IDT 5 and the output-side IDT 6 are arranged to be near to each other, a propagation loss, which is roughly obtained by multiplying a propagation loss per wavelength by a value equal to half of the average number of electrode fingers 3 arranged in both the input-side IDT 5 and the output-side IDT 6, is necessarily caused in the LN substrate 1.
For example, as shown in FIG. 3, in cases where each of the input-side IDT 5 and the output-side IDT 6 arranged to be near to each other is composed of seven electrode fingers 3, a propagation loss caused in the LN substrate 1 is roughly equal to three or four times of the propagation loss per wavelength shown in FIG. 1. For example, in cases where the propagation loss per wavelength is equal to 0.02 (dB/xcex), a propagation loss caused to the LN substrate 1 ranges from 0.06 to 0.08 dB.
That is, to obtain an SAW device having a low propagation loss, it is important to use an LN substrate 1 in which a low propagation loss per wavelength is caused. Therefore, lithium niobate (LN) processed at the cut angle xcex8 higher than 64 degrees is used for a conventional elastic wave device.
As is described above, the propagation loss per wavelength considerably influences on the insertion loss of the SAW filter. However, the influence on the insertion loss of the SAW filter is not limited to the propagation loss per wavelength in the LN substrate 1. That is, as material constants indicating characteristics of the LN substrate 1, an electro-mechanical coupling coefficient K2 relating to a transformation efficiency between electric signal and elastic wave, an electrostatic capacitance C0 relating to impedances of the input-side IDT 5 and the output-side IDT 6 and a propagation speed V of elastic wave are known as well as the propagation loss per wavelength. In these material constants, the electro-mechanical coupling coefficient K2 is important to determine the insertion loss of the SAW filter and a pass band width.
Also, the propagation of the surface acoustic wave in the case where the whole surface of the LN substrate 1 is covered with the electrode 2 as shown in FIG. 2 differs from that in the case where the electrode fingers 3 are periodically arranged on the LN substrate 1 as shown in FIG. 4. In practical use, the SAW filter has a structure in which the electrode fingers 3 are periodically arranged as shown in FIG. 4. Therefore, characteristics indicated by the practically-used SAW filter differ from those indicated by an SAW filter in which the whole surface of the LN substrate 1 is covered with the electrode 2 as shown in FIG. 2. Therefore, though the requirement for the material constants to obtain an elastic wave device indicating the most preferable characteristics are known in an elastic wave device using a pure surface acoustic wave such as Rayleigh wave or Bleustein-Gulyaev-Shimizu (BGS) wave for which no propagation loss is caused in principle, the requirement is not known in an elastic wave device using surface skimming bulk wave (SSBW) or leaky surface acoustic wave (LSAW).
Because the conventional elastic wave device has the above-described configuration, a conventional elastic wave device, in which the whole surface of the LN substrate 1 is covered with the electrode 2, is manufactured so as to minimize the propagation loss. However, a required condition for the material constants to make an elastic wave device using the SSBW or LSAW indicate the most preferable characteristics differs from a condition for minimizing the propagation loss. Therefore, there is a problem that the characteristics indicated by the conventional elastic wave device deteriorate because of the difference in the condition. That is, there is a problem that the insertion loss in the conventional elastic wave device deteriorates as compared with a minimum insertion loss required for practical use.
Also, though the conventional elastic wave device is manufactured so as to minimize the propagation loss, because the electro-mechanical coupling coefficient K2 considerably influencing on characteristics of the elastic wave device is not set to an optimum value, there is another problem that the insertion loss and/or pass band width in the conventional elastic wave device deteriorate.
An object of the present invention is to provide, with due consideration to the drawbacks of the conventional elastic wave device, an elastic wave device having characteristics of a broad band and a low loss.
An elastic wave device according to the present invention comprises a substrate of a piezo-electric element containing lithium niobate as a main component, and an electrode which is formed in a comb-like shape, is made of conductive material having a prescribed thickness and is arranged on the substrate. In particular, the elastic wave device is characterized in that a surface of the substrate is set to a plane which is obtained by rotating a plane perpendicular to a crystal Y-axis of the lithium niobate by an angle ranging from 55 degrees to 57 degrees around a crystal x-axis of the lithium niobate, and in that a duty ratio (w/p), which is determined according to both a width w of each of a plurality of electrode fingers composing the electrode and an arrangement interval p of each pair of electrode fingers, is equal to or higher than 0.4 and is lower than 1.0 for each electrode finger.
Therefore, there is an effect that an elastic wave device having characteristics of a broad band and a low loss as compared with those in the prior art can be obtained.
An elastic wave device according to the present invention comprises a substrate of a piezo-electric element containing lithium niobate as a main component, and an electrode which is formed in a comb-like shape, is made of conductive material having a prescribed thickness and is arranged on the substrate. In particular, the elastic wave device is characterized in that a surface of the substrate is set to a plane which is obtained by rotating a plane perpendicular to a crystal Y-axis of the lithium niobate by an angle ranging from 57 degrees to 62 degrees around a crystal x-axis of the lithium niobate, and in that a duty ratio (w/p), which is determined according to both a width w of each of a plurality of electrode fingers composing the electrode and an arrangement interval p of each pair of electrode fingers, is equal to or higher than 0.5 and is lower than 1.0 for each electrode finger.
Therefore, there is an effect that an elastic wave device having characteristics of a broad band and a low loss as compared with those in the prior art can be obtained.
An elastic wave device according to the present invention comprises a substrate of a piezo-electric element containing lithium niobate as a main component, and an electrode which is formed in a comb-like shape, is made of conductive material having a prescribed thickness and is arranged on the substrate. In particular, the elastic wave device is characterized in that a surface of the substrate is set to a plane which is obtained by rotating a plane perpendicular to a crystal Y-axis of the lithium niobate by an angle ranging from 62 degrees to 67 degrees around a crystal x-axis of the lithium niobate, and in that a duty ratio (w/p), which is determined according to both a width w of each of a plurality of electrode fingers composing the electrode and an arrangement interval p of each pair of electrode fingers, is equal to or higher than 0.6 and is lower than 1.0 for each electrode finger.
Therefore, there is an effect that an elastic wave device having characteristics of a broad band and a low loss as compared with those in the prior art can be obtained.
An elastic wave device according to the present invention comprises a substrate of a piezo-electric element containing lithium niobate as a main component, and an electrode which is formed in a comb-like shape, is made of conductive material having a prescribed thickness and is arranged on the substrate. In particular, the elastic wave device is characterized in that a surface of the substrate is set to a plane, which is obtained by rotating a plane perpendicular to a crystal Y-axis of the lithium niobate by an angle ranging from 67 degrees to 71 degrees around a crystal x-axis of the lithium niobate, and in that a duty ratio (w/p), which is determined according to both a width w of each of a plurality of electrode fingers composing the electrode and an arrangement interval p of each pair of electrode fingers, is equal to or higher than 0.7 and is lower than 1.0 for each electrode finger.
Therefore, there is an effect that an elastic wave device having characteristics of a broad band and a low loss as compared with those in the prior art can be obtained.
An elastic wave device according to the present invention comprises a substrate of a piezo-electric element containing lithium niobate as a main component, and an electrode which is formed in a comb-like shape, is made of conductive material having a prescribed thickness and is arranged on the substrate. In particular, the elastic wave device is characterized in that a surface of the substrate is set to a plane, which is obtained by rotating a plane perpendicular to a crystal Y-axis of the lithium niobate by an angle ranging from 71 degrees to 76 degrees around a crystal x-axis of the lithium niobate, and in that a duty ratio (w/p), which is determined according to both a width w of each of a plurality of electrode fingers composing the electrode and an arrangement interval p of each pair of electrode fingers, is equal to or higher than 0.8 and is lower than 1.0 for each electrode finger.
Therefore, there is an effect that an elastic wave device having characteristics of a broad band and a low loss as compared with those in the prior art can be obtained.